Micro-Electro-Mechanical Systems (MEMS) integrate mechanical elements, such as microsensors and microactuators, and electronics on a common substrate through the utilization of microfabrication technology. MEMS are typically micromachined using integrated circuit (IC) compatible batch-processing techniques that selectively etch away parts of a silicon wafer or add new structural layers. MEMS range in size from several micrometers to many millimeters. These systems sense, control, and actuate on a micro scale and function individually or in arrays to generate effects on a macro scale.
Microsensors, such as acceleration and shock sensors, are known in the prior art. While shock sensors come in many shapes and sizes, they typically involve the use of a suspended structure to detect vibration with peak excursions of that structure closing an electrical contact to indicate that a shock has occurred or a threshold has been exceeded. Acceleration sensors typically use a resonant structure to detect motion. One type of detector includes a silicon mass suspended by silicon beams with ion implanted piezoresistors on the beams to sense motion. Another type of detector uses capacitance changes to detect movement of the beam. Another type employs a shift in a physical load to produce a shift in the structure's resonant frequency.
A conventional shock sensor is shown in FIG. 1. Shock sensor 10 includes substrate 11, insulating layer 12, conductive cantilever 13 having a free end and fixed end, and contact conductor 14. Voltage is applied to the conductive cantilever 13 that serves as the top electrode. Contact conductor 14 serves as the bottom electrode. A shock with sufficient magnitude causes the free end of conductive cantilever 13 to touch contact conductor 14 completing the circuit. Current is detected by an ammeter (17). Once the circuit is completed, however, it immediately opens again. Thus, the detector must be continuously monitored to detect a shock. This can be a problem for one time use applications that require a sensor to disable itself when an extreme vibration is detected. Another problem is that the amplitude of a shock that causes the cantilever to close the electrical contact is fixed by the material properties and geometry of the sensor. Thus, variation of the detection threshold can only be made by physical modification of the distance between the electrodes.
An example of a conventional acceleration sensor is provided by U.S. Pat. No. 4,855,544. It discloses acceleration sensor 20 including a cantilevered beam 23 having an integral end mass at the free end of the beam as shown in FIG. 2. Prior art acceleration sensor 20 further includes substrate 21, insulating layer 22, and bottom conductor 24. Acceleration causes deflection of the free end of beam 23 from a relaxed condition. As acceleration increases, beam 23 will move to an increasingly strained condition with the free end moving towards bottom conductor 24. Movement of beam 23 is typically detected by a capacitance measurement. One problem with this prior art accelerometer is that there is no simple mechanism for threshold detection. Thus, it cannot automatically disable itself to eliminate large amplitude noise at a specific frequency.
In light of the foregoing, there is a need for bistable threshold sensors that can detect extremes in acceleration and that allow the detection threshold to be electrically modified.